THE CAMBRIAN PERIOD. 87 



The Trilobites vary much in size, and the Cambrian forma- 

 tion presents examples of both the largest and the smallest 

 members of the order. Some of the young forms may be little 

 bigger than a millet-seed, and some adult examples of the 

 smaller species (such as Agnostus) may be only a few lines in 

 length; whilst such giants of the order as Paradoxides and 

 Asaphus may reach a length of from one to two feet. Judging 

 from what we actually know as to the structure of the Trilo- 

 bites, and also from analogous recent forms, it would seem that 

 these ancient Crustaceans were mud-haunting creatures, deni- 

 zens of shallow seas, and affecting the soft silt of the bottom 

 rather than the clear water above. Whenever muddy sedi- 

 ments are found in the Cambrian and Silurian formations, 

 there we are tolerably sure to find Trilobites, though they are 

 by no means absolutely wanting in limestones. They appear 

 to have crawled about upon the sea-bottom, or burrowed in the 

 yielding mud, with the soft under surface directed downwards ; 

 and it is probable that they really derived their nutriment from 

 the organic matter contained in the ooze amongst which they 

 lived. The vital organs seem to have occupied the central lobe 

 of the skeleton, by which they were protected; and a series of 

 delicate leaf-like paddles, which probably served as respiratory 

 organs, would appear to have been carried on the under surface 

 of the thorax. That they had their enemies may be regarded 

 as certain; but we have no evidence that they were furnished 

 with any offensive weapons, or, indeed, with any means of 

 defence beyond their hard crust, and the power, possessed by 

 so many of them, of rolling themselves into a ball. An addi- 

 tional proof of the fact that they for the most part crawled 

 along the sea-bottom is found in the occurrence of tracks and 

 markings of various kinds, which can hardly be ascribed to 

 any other creatures with any show of probability. That this, 

 is the true nature of some of the markings in question cannot 

 be doubted at all ; and in other cases no explanation so prob- 

 able has yet suggested. If, however, the tracks which 

 have been described from the Potsdam Sandstone of North 

 America under the name of Protichnites are really due to the 

 peregrinations of some Trilobite, they must have been pro- 

 duced by one of the largest examples of the order. 



As already said, the Cambrian Rocks are very rich in the 

 remains of Trilobites. In the lowest beds of the series (Long- 

 mynd Rocks), representatives of some half-dozen genera have 

 now been detected, including the dwarf Agnostus and the giant 



